The present invention relates to motor vehicle tyres, particularly for lorries and the like, intended for heavy-haulage.
The invention is particularly concerned with tyres intended for use on the live axle of trucks or articulated lorries on mid-length journeys, i.e. on routes which, as is well known, impose severer demands and generate more wear than long journeys, which generally take place on motorways.
Tyres designed to be mounted on live axles usually have a tread of elastomeric material containing a relief pattern comprising circumferential grooves intersected by transverse grooves so as to form a plurality of blocks, hence the name xe2x80x9cblockxe2x80x9d tyres, to distinguish them from xe2x80x9cribbedxe2x80x9d tyres formed from continuous circumferential ribs having poorer grip properties.
In the remainder of the description and in the claims that follow, the expression xe2x80x9celastomeric materialxe2x80x9d is used to indicate the rubber compound in its entirety, that is the total mixture composed of one or more base polymers suitably amalgamated with reinforcing fillers and other product additives and process additives such as accelerators, retardants, anti-ageing agents, plasticizers, cross-linking agents and so forth.
Furthermore, the term xe2x80x9ctreadxe2x80x9d is used to indicate the strip of elastomeric material provided with a sheet of a different compound laid on the radially innermost surface of the said strip to promote the adhesion of the tread when cold to the underlying elements of the carcass, in particular the belt layers.
Tyres for live axles have to satisfy multiple requirements: in particular they must provide good grip on both dry surfaces and wet and/or snow-covered surfaces.
In addition, both the market and many national legislations are increasingly demanding quiet-running tyres, whereas the use of xe2x80x9cblockxe2x80x9d tyres generally produces more noise than xe2x80x9cribbedxe2x80x9d tyres.
The tread must also wear evenly and offer high mileage.
On top of this, treads are required to have a pattern capable of efficiently expelling stones and small fragments that become trapped in the grooves of the tread, since if such stones or fragments were to remain in the tread they could perforate the tread as the tyre wears down and so damage the underlying belt structure.
The prior art already includes tyres capable of confronting these problems and offering a solution to them.
For example, Italian Patent 1,245,773 by the Applicant discloses a tread comprising at least four rows of blocks defined by at least three circumferential grooves of different widths, the axially innermost groove being wider and the two lateral grooves on either side of the equatorial plane of the tyre being narrower, and by a plurality of oblique transverse grooves connecting pairs of circumferentially adjacent grooves.
In this solution the narrow groove that separates the two lateral rows of blocks on each half of the tyre is characterized by the fact that its width is no greater than 2.5 mm and its depth not greater than that of the confluent transverse grooves while the blocks of the two lateral rows are circumferentially staggered relative to each other so that every block of each row is next to two consecutive blocks of the adjacent row.
This patent describes other solutions comprising, for example, two additional axially internal circumferential grooves defining a pair of circumferential ribs, in the walls of which there are oblique transverse grooves extending as far as the equatorial plane, so that the ribs have the appearance of two adjacent rows of blocks.
The blocks of all rows have an ordered configuration, the longitudinal sides of the blocks of any one row lying along two lines parallel to the equatorial plane.
In another solution the two rows of blocks of the two central ribs can be joined together to form a single row straddling the equatorial plane and in another solution the blocks of the central row may be of roughly hexagonal shapes.
Also known is a tyre comprising a xe2x80x9cblockxe2x80x9d-type tread composed of four, that is two lateral and two central, circumferential grooves of equal width, and a plurality of transverse grooves that together define five rows of blocks, that is two axially external shoulder rows, two axially internal intermediate rows and a central row of blocks passing through the equatorial plane of the tyre.
The blocks of the central row and of the intermediate rows are all of identical xe2x80x9cZxe2x80x9d shapes defined by two longitudinal sides and two transverse end sides, while the shoulder blocks are defined by two longitudinal sides and two transverse end sides, one at the front and one at the rear, both in the form of arrows and complementary to each other.
The longitudinal sides of the blocks of all rows are parallel with each other and inclined to the equatorial plane while the transverse end sides of the blocks of the central and intermediate rows are parallel with each other and slightly inclined to a plane perpendicular to the equatorial plane.
The blocks of the adjacent rows are also circumferentially offset relative to each other.
The Applicant was convinced that the performance of tyres of the prior art with xe2x80x9cblockxe2x80x9d-type treads could be improved, particularly as regards protection against the penetration of stones towards the belt in the central zone of the tread, which is usually at greater risk, without this improvement triggering negative changes to other characteristics such as resistance to stone penetration in the lateral zones of the tread, grip, wear resistance and quiet running, but instead if possible introducing further improvements.
It was thought that it would be possible to achieve this result by employing a xe2x80x9cblockxe2x80x9d-type tread comprising a widening of the central circumferential grooves disposed between the central row and the intermediate rows of blocks while nonetheless maintaining the width of the lateral circumferential grooves within the range of a predetermined value that would still be useful in protecting against the penetration by stones.
It was realized that the accomplishment of this task could depend on achieving a particular ratio between the widths of the circumferential, lateral and central grooves.
It was then felt that although the reduction in the width of the blocks of the intermediate rows, necessary to permit both widening of the central grooves and a predetermined width of the lateral grooves, would introduce a consequent poorer rigidity of the intermediate blocks, this effect could be compensated for by increasing the rigidity of the blocks of the central rows and by providing an elastic linkage between the blocks of the shoulder and intermediate rows.
The idea, in fact, was to liken the central, intermediate and shoulder rows of blocks to three parallel springs under load so that by giving the blocks of the central rows a more rigid shape than that of the other blocks it was possible to create in the central spring a greater elastic reaction which would protect the other rows of blocks.
It was also felt that it should be possible to reinforce the resistance to bending deformations of the intermediate and shoulder rows by means of a mutual elastic linkage so that these rows in combination could present an increased resistance to bending deformations.
In order to achieve satisfactorily quiet running the characteristics of inclining all the longitudinal sides of the blocks to the equatorial plane, and the circumferential staggering of the blocks of one row relative to the next, have been maintained.
In an initial aspect the invention therefore relates to a vehicle wheel tyre comprising a carcass structure that includes a central crown portion and two sidewalls terminating in two beads for fixing the tyre to a wheel rim; a belt structure coaxially fixed to the carcass structure; and a tread extending coaxially around the belt structure, its thickness being defined between the outer surface intended to contact the ground and the inner surface in contact with the said belt structure, having a tread pattern which comprises four, that is two lateral and two central, continuous circumferential grooves and a plurality of transverse grooves that together define five circumferential rows of blocks, that is two axially external shoulder rows, two axially internal intermediate rows and a central row that basically straddles the equatorial plane of the tyre, each block being a polygon composed of two sides extending longitudinally in a circumferential direction and two transverse end sides, one at the front and one at the rear with reference to a direction of travel, characterized in that in the said tread pattern:
a) the longitudinal sides of the blocks of each row are inclined in the same direction with a predetermined angle xe2x80x9cxcex1xe2x80x9d to the equatorial plane of the tyre,
b) the blocks of one row are circumferentially staggered relative to the blocks of the next row,
c) the ratio of the width, measured axially, of each central circumferential groove to the width, measured axially, between opposite longitudinal sides of the blocks of the central row is not less than 20%,
d) the ratio of the width of the lateral circumferential groove to the width of the central circumferential groove is between 45% and 60%,
e) there are means of stiffening the blocks of the central row consisting in the fact that each front and rear side of each block is formed by a first and a second straight segment separated by an intermediate third connecting segment, the said first and second segments being inclined to the equatorial plane, and the profile of the front side, rotated through 180xc2x0, being the same as that of the rear side, and
f) there are circumferential elastic means of linkage between the blocks of the shoulder and intermediate rows.
Conveniently, for the purposes of improved noise reduction, the inclination of said third segment is the opposite of that of the first and second segments.
The elastic linkage means preferably consist in the presence of elastomeric material of predetermined height between the bottom line of the axially external lateral groove and the inner boundary surface of the tread, and in an axially outer lateral groove whose width, measured on the outer boundary surface of the tread, is at least 6.5 mm.
The tyre conveniently comprises elastic linkage means between the adjacent blocks of the central row to contain the bending deformations; these elastic linkage means consist in the fact that, between pairs of adjacent blocks along the relevant transverse groove, an elastomeric relief extends from the internal surface of the tread toward the outer surface up to a predetermined height, preferably equal to at least 15% of the thickness of the tread.
Also, in a preferred embodiment, the first and second segments of both front and rear sides of the central blocks are inclined in the same direction by a predetermined angle xe2x80x9cxcex2oxe2x80x9d relative to a plane perpendicular to the equatorial plane of the tyre.
Preferably, the said angle xe2x80x9cxcex2oxe2x80x9d is between 15xc2x0 and 21xc2x0.
Still more preferably, in each central block the intermediate third segment between the two straight segments forms an angle xe2x80x9cxcex3xe2x80x9d of between 30xc2x0 and 60xc2x0 to a plane perpendicular to the equatorial plane.
The said constructions of the transverse end sides of the central blocks advantageously improve the characteristics of quiet running and grip.
In accordance with one particular embodiment the tyre of the invention comprises elastic linkage means between adjacent blocks of the shoulder rows.
These elastic linkage means preferably consist in the fact that, between pairs of adjacent blocks along the relevant transverse groove, an elastomeric relief extends from the internal surface of the tread toward the outer surface up to a predetermined height.
The invention advantageously involves determining the height of this relief according as greater or less stiffness is required.
This relief can therefore vary between 9% and 60% of the thickness of the tread.
The tyre with the pattern according to the invention may comprise stone expulsion means, preferably comprising a narrowing of the grooves on only one of the defining sides of the grooves.
In another preferred embodiment the tyre comprises in a predetermined pitch of its pattern five transverse grooves, the distance between the centre lines of any two of the five transverse grooves being approximately ⅕ of the pitch.